Comparison of the active sites of CYP2A6 I208S/I300F/G301A/G369S with phenacetin (green, PDB 3EBS), CYP2A6 (purple, PDB 1Z10), and CYP2A13 (orange, PDB 2P85). The four residues mutated in the CYP2A6 quadruple mutant and identical to the residues found in CYP 2A13 are labeled in bold. The hydrogen bond from phenacetin to N297 is shown as a dashed black line.